patch 8.2.1502: Vim9: can use += with a :let command at script level

Problem:    Vim9: can use += with a :let command at script level.
Solution:   Give an error.
diff --git a/src/version.c b/src/version.c
index 4e7b9ad..62692b8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1502,
+/**/
     1501,
 /**/
     1500,